1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an outdoor liquid crystal display device that has a transparent protection plate provided on a display face thereof.
2. Description of the Background Art
In a liquid crystal display device for outdoor installation, it is necessary to protect a liquid crystal panel from an external impact and water immersion caused by rain. Therefore, a display face of the liquid crystal panel is covered with a transparent protection plate made of resin or glass, and a waterproof finish is provided on a peripheral portion of the transparent protection plate.
Where a transparent protection plate is provided on the display face of the liquid crystal panel, an air space including air bubbles is normally formed between the liquid crystal panel and the transparent protection plate. Due to the air space, external light incident on the display face is reflected by the rear face of the transparent protection plate and the surface of the liquid crystal panel. As a result, visibility becomes lower.
To counter this problem, Japanese Patent Application Laid-Open No. 2005-316042 discloses a liquid crystal display device that has a transparent resin or the like filling a space between the liquid crystal panel and the transparent protection plate, and a liquid crystal display device that has a transparent adhesive material provided in the space.
By those conventional techniques, it is possible to increase the visibility of the display by reducing generation of air bubbles, but air bubbles still exist. Therefore, it is difficult to bond the transparent protection plate onto the liquid crystal panel without air bubbles. Particularly, after a liquid crystal display device is formed by combining a liquid crystal panel, a planar light source unit, and a front frame, it is even more difficult to bond the transparent protection plate onto the liquid crystal panel for the below reasons.
First, by the transparent resin filling technique disclosed in Japanese Patent Application Laid-Open No. 2005-316042, the resin enters a rear face side of the liquid crystal panel through a space between the liquid crystal panel and the front frame holding the liquid crystal panel. When the resin reaches the planar light source unit, unevenness in display is caused. To avoid this problem, a sealing member or the like to prevent the resin from entering the space between the front frame and the liquid crystal panel needs to be formed, and the number of manufacturing procedures increases accordingly.
By a technique of bonding the transparent protection plate onto the liquid crystal panel with the transparent adhesive material, on the other hand, it is necessary to press the display face side of the liquid crystal panel with the front frame, so as to bond the transparent protection plate onto the liquid crystal panel, with no air bubbles entering the space between the liquid crystal panel and the transparent protection plate. However, when a center of the transparent protection plate bonding face of the liquid crystal panel is pressed in a structure having the planar light source unit supporting the peripheral portion of the liquid crystal panel, a problem occurs in that the liquid crystal panel is bent and air bubbles easily enter the bonding face. Also, in a liquid crystal display device having a liquid crystal panel in close contact with a planar light source unit, an optical sheet of the planar light source unit is pressed when the transparent protection plate bonding face of the liquid crystal panel is pressed, and the unevenness in display is caused.
To solve the above problems, there is proposed a liquid crystal display device that is formed by bonding a transparent protection plate to a liquid crystal panel with a transparent elastic adhesive material before the liquid crystal panel, a planar light source unit, and a front frame are incorporated into the structure.
In such a liquid crystal display device, the front frame needs to be incorporated into the structure after the transparent protection plate is bonded to the liquid crystal panel. Therefore, the transparent protection plate is housed in the front frame. In this structure, it is necessary to provide a waterproof finish inside an opening end portion of the front frame. The waterproof finish may be provided by performing the bonding with a caulking material, or applying pressure with an O-ring, or the like. To achieve high productivity, it is preferable to apply pressure with an O-ring.
However, when end portions of the transparent protection plate are pressed by an O-ring inside the opening end portion of the front frame, a stress is transmitted to the liquid crystal panel, since the transparent protection plate is held only by the liquid crystal panel. As a result, the unevenness is caused in the display of the liquid crystal panel.